Fuel diffusion unit, fuel supply unit, and fuel cell system including the same

ABSTRACT

A fuel diffusion unit including: a fuel diffusion plate; a diffusion sheet disposed on fuel diffusion plate, to evenly distribute a fuel to the fuel diffusion plate; a primary transportation unit disposed on the diffusion sheet; secondary transportation units connected to the primary transportation unit, to distribute the fuel to the fuel from the primary transportation unit to the diffusion sheet. The diffusion sheet has a wetting direction that allows the fuel to flow in a predetermined direction. The fuel diffusion unit can be included in a fuel supply unit and a fuel cell system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Application No.2007-140669, filed Dec. 28, 2007, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a fuel supply unit, and moreparticularly, to a fuel supply unit that can vaporize a fuel, and a fuelcell system that employs the fuel supply unit.

2. Description of the Related Art

A fuel cell is an electricity generator that chemically reacts anoxidant and a fuel, to produce electricity. A fuel cell can continuouslygenerate electricity, as long as a fuel is supplied.

A direct methanol fuel cell (DMFC) is a fuel cell that generateselectricity, through a reaction between methanol supplied to an anode,and oxygen supplied to a cathode. In a DMFC, the anode and the cathodeface each other across an electrolyte membrane. An assembly of theanode, the cathode, and the electrolyte membrane is conventionallyreferred to as a membrane and electrode assembly (MEA).

The cathode is exposed to an oxidant, such as oxygen in air. The anodeis surrounded by a spacer, and vaporized methanol is supplied to theanode, through a plurality of holes formed in the spacer. Electrons aregenerated at the anode, and the electrons move through an electricalcircuit, to the cathode. At this point, when a load is applied to thecircuit, work can done using the generated electricity.

A fuel cell is generally part of a fuel cell system. A fuel cell systemcan be classified as an active-type, or a passive-type, according to howa fuel is supplied to an anode. In the active-type, a low concentrationliquid methanol is pumped to the anode.

Passive-type fuel cell systems can include a fuel supply unit. The fuelsupply unit vaporizes liquid methanol stored in a cartridge, and thenthe vapor is supplied to the anode, at room temperature, by diffusion.Passive-type fuel cell systems are often used in mobile devices, becausepassive-type fuel cell systems generally are smaller in size.

It is generally beneficial for a fuel supply system to evenly supplyfuel to an entire surface of an anode. In this way, a rapid initialstart-up can be realized, by uniformly supplying vaporized fuel to anMEA.

If the fuel is supplied through a single supply path, it takes time toachieve a uniform concentration of fuel across the anode. Thus, aninitial start-up may be delayed. Therefore, there is a need to develop aconfiguration of fuel supply system that can uniformly supply fuelacross an anode of an MEA, in a short time.

SUMMARY OF THE INVENTION

To address the above and/or other problems, aspects of the presentinvention provide a fuel diffusion unit, and a fuel supply unit, whichcan realize a rapid initial start-up, by uniformly supplying fuel to amembrane electrode assembly (MEA), and a fuel cell system employing thesame.

According to an aspect of the present invention, there is provided afuel diffusion unit comprising: a fuel diffusion plate; a diffusionsheet disposed on the fuel diffusion plate, to evenly distribute fuel tothe fuel diffusion plate; a primary transportation unit disposed on thediffusion sheet; and secondary transportation units connected to theprimary transportation unit, to distribute the fuel from the primarytransportation unit to the diffusion sheet. The fuel flows through thediffusion sheet in a wetting direction of the diffusion sheet.

According to an aspect of the present invention, there is provided afuel supply unit for a fuel cell, comprising: a fuel storage unit; afuel diffusion plate to evaporate fuel supplied from the fuel storageunit; a diffusion sheet disposed on the fuel diffusion plate, touniformly distribute the fuel to the fuel diffusion plate, having awetting direction along which the fuel is distributed; a primarytransportation unit disposed on the diffusion sheet, connected to thefuel storage unit; an actuator to pump the fuel through the primarytransportation unit, and secondary transportation units that areconnected to the primary transportation unit, to distribute the fuel tothe diffusion sheet.

According to an aspect of the present invention, there is provided afuel cell system comprising: a unit cell to generate electricity using afuel; a fuel storage unit to store the fuel; a fuel diffusion plate tovaporize the fuel, and to supply the vaporized fuel to the unit cell; adiffusion sheet to uniformly distribute the fuel to the fuel diffusionplate, having a wetting direction along which the fuel is distributed; aprimary transportation unit disposed on the diffusion sheet, andconnected to the fuel storage unit; secondary transportation unitsconnected to the primary transportation unit, to distribute the fuel tothe diffusion sheet; and an actuator to pump the fuel through theprimary transportation unit.

Additional aspects and/or advantages of the invention will be set forth,in part in the description which follows, and in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent, and more readily appreciated, from the following descriptionof the exemplary embodiments, taken in conjunction with the accompanyingdrawings, of which:

FIG. 1 is a plan view of a fuel supply unit for a fuel cell, accordingto an exemplary embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1;

FIG. 3 is a drawing showing wetting directions in diffusion sheets ofFIG. 2;

FIG. 4 is a cross-sectional view of a fuel cell system that employs thefuel supply unit of FIG. 1, according to an exemplary embodiment of thepresent invention;

FIG. 5 is a cross-sectional view of a configuration of a cell of FIG. 4;and

FIG. 6 is a graph showing a cell voltage, according to time andtemperature measured in multiple positions of cells that use a fuelsupply unit, according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The exemplary embodiments are described below, in order toexplain the aspects of the present invention, by referring to thefigures.

FIG. 1 is a plan view of a fuel supply unit 100 for a fuel cell,according to an exemplary embodiment of the present invention. FIG. 2 isa cross-sectional view taken along line A-A of FIG. 1, and FIG. 3 is adrawing for explaining wet directions in diffusion sheets of FIG. 2.

Referring to FIGS. 1 through 3, the fuel supply unit 100 includes: afuel storage unit 110 that stores fuel; a fuel diffusion unit 140; andan actuator 120 that is connected between the fuel storage unit 110 andthe fuel diffusion unit 140. The fuel is stored in the fuel storage unit110, as a liquid. The fuel may be, for example, methanol MeOH, ethanolEtOH, or sodium boromohydride NaBH₄.

The fuel diffusion unit 140 includes a fuel diffusion plate 130 tovaporize the fuel, a primary transportation unit 141, secondarytransportation units 142, and a diffusion sheet 144. The primarytransportation unit 141 can be a tube that extends in a lengthwisedirection along the center of the fuel diffusion plate 130. An end ofthe primary transportation unit 141 is connected to the actuator 120,and the other end is blocked. The primary transportation unit 141 mayhave a diameter φ1, of 0.1 to 1 mm, in consideration of the portabilityand mobility of a mobile device in which a fuel cell is used. That is,the above range of diameters achieve a stable fuel supply to the fueldiffusion plate 130, from the fuel storage unit 110, without beingsignificantly affected by the orientation of the primary transportationunit 141. The primary transportation unit 141 may have a larger diameterthan the secondary transportation units 142.

The secondary transportation units 142 can be small tubes, such ascapillaries, and can be disposed on opposing sides of the primarytransportation unit 141. The secondary transportation units 142 haveattached ends that are connected to the primary transportation unit 141,and open ends through which the fuel can exit the secondarytransportation units 142. The attached ends can be offset from oneanother, along the length of the primary transportation unit 141, suchthat the attached ends do not face one another, across the primarytransportation unit 141.

The secondary transportation units 142 include first tubes 142 a andsecond tubes 142 b. The first tubes 142 a can be straight, such that theopen ends of the first tubes 142 a face away from the primarytransportation tube 141. The second tubes 142 b can be curved, such thatthe open ends of the second tubes 142 b generally face toward theprimary transportation unit 141. The first tubes 142 a and the secondtubes 142 can be alternately disposed along each side of thetransportation unit 141. In other words, different types of thesecondary transportation units 142 are adjacent to one other, on eachside of the primary transportation unit 141.

The different secondary transportation units 142 facilitate a rapid anduniform delivery of fuel, across the entire fuel diffusion plate 130.The structures of the secondary transportation units 142 are not limitedto the shapes depicted in FIG. 1. In other words, the secondarytransportation units 142 can have various shapes, orientations, and/orlengths. For example, the second tubes 142 b can be straight, and can beshorter than the first tubes 142 a.

The secondary transportation units 142 may have increasing diameters,the further each secondary transportation unit 142 is from the actuator120. The change in diameter facilitates a uniform supply of fuel to theentire fuel diffusion plate 130, since a fuel supply pressure decreasesas a distance from the actuator 120 increases. The diameter φ2 of thesecondary transportation units 142 may be in a range from 5 to 250 μm.

The secondary transportation units 142 can be connected to the primarytransportation unit 141, in various ways. For example, if the primarytransportation unit 141 is formed from a flexible material, and thesecondary transportation units 142 are formed from a harder material,the secondary transportation unit 142 can be stabbed into the primarytransportation unit 141. The secondary transportation units 142 can bewelded or glued to the primary transportation unit 141, for example.

The number of the secondary transportation units 142 can be determined,according to the area of the fuel diffusion plate 130. For example, ifthe fuel diffusion plate 130 is relatively large, the number of thesecondary transportation units 142 can be increased, to increase theamount of fuel flowing there through. If the number of the secondarytransportation units 142 is increased in the same area, the amount offuel supply per unit area can also be increased. Thus, the fueldiffusion plate 130 can be more rapidly supplied with fuel, as comparedto a conventional fuel diffusion plate, in which fuel is suppliedthrough a single path, since the fuel supply unit 100 supplies the fuelsimultaneously through the secondary transportation units 142.

The diffusion sheet 144 is formed to completely cover a surface of thefuel diffusion plate 130. The diffusion sheet 144 can, in some exemplaryembodiments, include a first diffusion sheet 144 a and a seconddiffusion sheet 144 b. The diffusion sheet 144 may have a wettingdirection. Due to the wetting direction, the diffusion sheet 144 canrapidly transport fuel from the secondary transportation units 142, tothe entire surface of the fuel diffusion plate 130, by absorbing andtransporting the fuel. For example, the diffusion sheet 144 cantransport fuel by capillary action, wicking, or the like.

The second diffusion sheet 144 b is positioned under the first diffusionsheet 144 a. As depicted in FIG. 3, the first diffusion sheet 144 a andthe second diffusion sheet 144 b have wetting directions that aredifferent from each other. The different wetting directions, of thefirst diffusion sheet 144 a and the second diffusion sheet 144 b,facilitate the rapid distribution of fuel to the entire surface of thefuel diffusion plate 130. The wetting direction of the first diffusionsheet 144 a may be generally perpendicular to that of the seconddiffusion sheet 144 b, or may be otherwise angled from the wettingdirection of the second diffusion sheet 144 b. Accordingly, thediffusion sheet 144 can rapidly distribute fuel supplied from thesecondary transportation units 142, to the entire surface of the fueldiffusion plate 130, by absorbing and transporting the fuel.

FIG. 4 is a cross-sectional view of a fuel cell system 200 having thefuel supply unit 100, according to an exemplary embodiment of thepresent invention. FIG. 5 is a cross-sectional view of a configurationof a unit cell 150, of FIG. 4. Referring to FIGS. 4 and 5, the fuel cellsystem 200 includes the fuel supply unit 100 of FIG. 1, and the unitcell 150, which is connected to the fuel supply unit 100, to generate acurrent.

The unit cell 150 includes an anode 152, a cathode 154, and anelectrolyte membrane 153 interposed between the anode 152 and thecathode 154. The cathode 154 is exposed to the outside, so that thecathode 154 can be supplied with an oxygen source (air). The anode 152is surrounded by a spacer 155. Fuel is evaporated from the fueldiffusion plate 130, and is supplied to the anode 152, through aplurality of supply holes 155 a formed in the spacer 155.

As shown below, the Chemical Equation 1 occurs at the anode 152, togenerate electrons, and the electrons move to the cathode 154, along acircuit 156, to participate in the Chemical Equation 2. A load 157 canapplied to the circuit 156. An assembly of the anode 152, the cathode154, and the electrolyte membrane 153 is conventionally referred to as amembrane and electrodes assembly (MEA).

CH₃OH+H₂⇄CO₂+6H++6e−  [Chemical Equation 1]

3/2O₂+6H++6e−⇄3H₂O  [Chemical Equation 2]

Generally, the current generated from a single unit cell 150 is notlarge enough to use, and thus, a plurality of the unit cells 150 areconnected in a series, on the fuel diffusion plate 130. Thus, a desiredvoltage/current can be obtained, by using multiple unit cells 150.

FIG. 6 is a graph showing unit cell voltages, according to time, andtemperatures of the unit cells, when the unit cells are supplied withfuel using the fuel supply unit 100. Referring to FIG. 6, the horizontalaxis indicates time in minutes, the vertical axis on the left sideindicates voltages V of the unit cells, and the vertical axis on theright side indicates cathode surface temperatures, in ° C. FIG. 6 showsthe voltage increase in each of the unit cells, when sixteen unit cellsare installed on the fuel diffusion plate 130 of the fuel supply unit100.

Referring to FIG. 6, the voltages of the sixteen unit cells uniformlyincrease, without a large deviation. This indicates that fuel isuniformly supplied to each of the unit cells, since the voltagesincrease in nearly the same manner. Thus, the fuel supply unit 100uniformly distributes fuel to the entire surface of the fuel diffusionplate 130, in a short time.

FIG. 6 also shows temperatures measured by three temperature detectingdevices that are installed in three different positions. It can be saidthat temperature nearly uniformly rises, although there is a slighttemperature deviation (approximately within 2° C.). This result denotesthat the amount of fuel cross-over is similar in each of the cells, andfuel was uniformly supplied to all of the sixteen cells. Thus, it isseen that the fuel supply unit 100 uniformly distributes fuel to theentire surface of the fuel diffusion plate 130, in a short time.

Although a few exemplary embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these exemplary embodiments, withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

1. A fuel diffusion unit comprising: a fuel diffusion plate to vaporizea fuel; a diffusion sheet to distribute a fuel across a surface of thefuel diffusion plate, having a wetting direction along which the fuel isdistributed; a primary transportation unit disposed on the diffusionsheet; and secondary transportation units connected to the primarytransportation unit, to distribute the fuel from the primarytransportation unit to the diffusion sheet.
 2. The fuel diffusion unitof claim 1, wherein the primary transportation unit is a tube having afirst end that is connected to a fuel storage unit, and a second endthat is blocked.
 3. The fuel diffusion unit of claim 2, wherein theprimary transportation unit has a diameter of 0.1 to 1 mm.
 4. The fueldiffusion unit of claim 1, wherein: the secondary transportation unitscomprise first tubes that are generally straight, and second tubes thatare curved; and the first and second tubes are alternately disposedalong each of two opposing sides of the primary transportation unit. 5.The fuel diffusion unit of claim 4, wherein the secondary transportationunits have diameters of 5 to 250 μm.
 6. The fuel diffusion unit of claim1, further comprising a plurality of the diffusion sheets that havedifferent wetting directions.
 7. A fuel supply unit for a fuel cell,comprising: a fuel storage unit to store a fuel; a fuel diffusion plateto vaporize the fuel; a diffusion sheet to distribute the fuel across asurface of the fuel diffusion plate, having a wetting direction alongwhich the fuel is distributed; a primary transportation unit disposed onthe diffusion sheet, and connected to the fuel storage unit; an actuatorto pump the fuel through the primary transportation unit; and secondarytransportation units connected to the primary transportation unit, todistribute the fuel to the diffusion sheet.
 8. The fuel supply unit ofclaim 7, wherein the primary transportation unit is a tube having afirst end that is connected to the fuel storage unit, and a second endthat is blocked.
 9. The fuel supply unit of claim 8, wherein the primarytransportation unit has a diameter of 0.1 to 1 mm.
 10. The fuel supplyunit of claim 7, wherein: the secondary transportation units comprisefirst tubes that are generally straight, and second tubes that arecurved; and the first and second tubes are alternately disposed alongeach of two opposing sides of the primary transportation unit.
 11. Thefuel supply unit of claim 10, wherein the secondary transportation unitshave a diameter of 5 to 250 μm.
 12. The fuel supply unit of claim 7,further comprising a plurality of the diffusion sheets that havedifferent wetting directions.
 13. A fuel cell system comprising: a unitcell to generate a current using a fuel; a fuel storage unit to storethe fuel; a fuel diffusion plate to vaporize the fuel, and to supply thevaporized fuel to the unit cell; a diffusion sheet to distribute thefuel across a surface of the fuel diffusion plate, having a wettingdirection along which the fuel is distributed; a primary transportationunit disposed on the diffusion sheet, and connected to the fuel storageunit; an actuator to pump the fuel through the primary transportationunit; and secondary transportation units that are connected to theprimary transportation unit, to distribute the fuel to the diffusionsheet.
 14. The fuel cell system of claim 13, wherein the primarytransportation unit is a tube having a first end that is connected to afuel storage unit, and a second end that is blocked.
 15. The fuel cellsystem of claim 14, wherein the primary transportation unit has adiameter of 0.1 to 1 mm.
 16. The fuel cell system of claim 13, wherein:the secondary transportation units comprise first tubes that aregenerally straight, and second tubes that are curved; and the first andsecond tubes are alternately disposed along each of two opposing sidesof the primary transportation unit.
 17. The fuel cell system of claim16, wherein the secondary transportation units have a diameter of 5 to250 μm.
 18. The fuel cell system of claim 13, further comprising aplurality of the diffusion sheets that have different wettingdirections.
 19. The fuel diffusion unit of claim 1, wherein: thesecondary transportation units comprise tubes that are spaced along thelength of the primary transportation unit; and the diameters of thetubes vary according to each tubes position on the primarytransportation unit.
 20. The fuel diffusion unit of claim 1, wherein thesecondary transportation units comprise tubes that that extend away fromthe primary transportation unit, and are offset from one another onopposing sides of the primary transportation unit.
 21. The fueldiffusion unit of claim 4, wherein: the first tubes have open ends thatface away from the primary transportation unit; and the second tubeshave open ends that face towards the primary transportation unit. 22.The fuel diffusion unit of claim 1, wherein: the secondarytransportation units comprise first and second tubes that extend awayfrom opposing sides of the primary transportation unit; and the firsttubes are longer than the second tubes.